The invention relates to a method of improving the dynamic strength of wheel disks of vehicle wheels made of heat-treatable alloys, as well as to apparatus for carrying out said method.
Aluminum vehicle wheels have a number of positive properties compared to conventional steel wheels. For example, they have a lower weight, better heat dissipation during braking, better balancing, and a more attractive appearance. Therefore, aluminum wheels have found increasingly wide application both in passenger and in commercial vehicles, in the latter case particularly in the United States.
The extent to which the weight of a wheel can be reduced, e.g., by using a thinner aluminum material, is limited by the dynamic strength of aluminum, which can be much lower than the strength measured under static conditions. Therefore, these are clear limits to the strength of worked material employed for wheels. This is especially applicable to a wheel disk, i.e., the central part of the wheel, which sometimes is subjected to considerable rotating of deflecting forces. Thus, the fatigue strength of aluminum determines to a large extent the dimensions and thereby the weight of the wheel. Up to a certain extent, the fatigue strength can be increased by using suitable alloys, but the gain is often insignificant.
Solution heat treatment such as, for example, described in "Robert B. Ross: Handbook of Metal Treatments and Testing, London", pp. 206-208, is known for improving the properties of worked material of various alloys, including aluminum alloys.
The use of solution heat treatment of wheel rims is also described in U.S. Pat. No. 3,172,787, in which wheel rims are manufactured from a continuously cast aluminum profile by appropriate rounding and welding. After completion of the welding seam, the rims are solution heat treated and then calibrated to the finished size and configuration. However, this prior art method includes no reference to the treatment of wheel disks in terms of their fatigue strength.
On the basis of the loads to which a wheel is subjected, the wheel disk i.e., the central part of the wheel, is manufactured from a work material which is approximately twice as strong as that of the rim. For the most common vehicle wheels, the strength of the wheel disk lies within the 7 to 14 mm range. During cooling or quenching in accordance with the solution heat treatment, the rim is cooled faster than the disk because of the thinner wall. The cooling in accordance with the solution heat treatment from approximately 550.degree. C. to room temperature creates a relatively large heat leakage during the cooling. If this cooling is carried out without control, considerable stresses may occur as a result of a nonuniform cooling rate. These stresses are added to the dynamic loads to which the wheel is normally subjected.